1. Field of the Invention
The present invention relates to a starting circuit of a passenger protecting apparatus such as air bag or seat belt pretensioner, which is actuated by detecting collision of a vehicle.
2. Description of the Prior Art
FIG. 9 illustrates a starting circuit of a conventional passenger protecting apparatus which is disclosed in, for example, Japanese Utility Model Application Laid-Open No. 55-63245. In FIG. 9, reference numeral 1 refers to a DC power source such as a battery for use in a vehicle, numeral 2 refers to an ignition switch serially connected to the DC power source 1, and numerals 3 and 4 refer to acceleration sensors (hereafter referred to as G sensors) having mechanical contacts 3b, 4b which are closed by collision. Further, reference numerals 3a, 4a refer to resistors of about 1 k.OMEGA., which are connected in parallel with the mechanical contacts 3b and 4b of the G sensors 3, 4 so as to feed slight monitor current in case the mechanical contacts are open. Reference numerals 10, 20 and 30 refer to squibs having resistances of several ohms for actuating the passenger protecting apparatus, and numerals 11, 21 and 31 refer to diodes serially connected to the squibs 10, 20 and 30, respectively. Reference numeral 12 refers to an auxiliary power supply for supplying the squibs 10, 20 and 30 with current in case a supply of current from the DC power source 1 is interrupted when a collision occurs. The auxiliary power supply 12 includes a resistor 12a, a capacitor 12b, and a diode 12c.
The operation will now be described. The G sensor 4 is referred to as a front G sensor because it is mounted in a front portion of a vehicle. The G sensor 4 is set such that the mechanical contact 4b is closed only when the G sensor 4 actually senses impact to start the passenger protecting apparatus. On the other hand, the G sensor 3 mounted in a vehicle cabin is referred to as a safety G sensor, and is set such that the mechanical contact 3b can be closed by even relatively soft impact. When the mechanical contacts 3b and 4b of the G sensors 3 and 4 are in a closed condition simultaneously, the DC power source 1 or the auxiliary power supply 12 supplies ignition current to the squibs 10, 20, and 30 so as to actuate the passenger protecting apparatus.
The starting circuit of the conventional passenger protecting apparatus is provided as set forth hereinbefore. That is, the plurality of squibs 10, 20 and 30 are connected to a common feeding circuit in parallel with each other. Accordingly, there are problems in that the resistances of the squibs may be unbalanced, and the passenger protecting apparatus may not start since the ignition current can not flow uniformly if any one of the squibs is short-circuited.